Pigment-pigment interactions in PCP of Amphidinium carterae investigated by nonlinear polarization spectroscopy in the frequency domain
Maria Krikunova 1, Heiko Lokstein 2, Dieter Leupold 1, Roger G. Hiller 3 and Bernd Voigt 1*
1 Max-Born-Institut
2 Institut für Biologie, Humboldt-Universität
3 Department of Biological Sciences, Macquarie-University, Sydney, Australia 2109
* To whom correspondence should be addressed. E-mail: bevoigt{at}rz.uni-potsdam.de.
Submitted on October 29, 2004
Revised on December 19, 2004
Accepted on 27 September 2005
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Abstract |
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Peridinin-chlorophyll a-protein (PCP) is a unique antenna complex in dinoflagellates that employs peridinin (a carotenoid) as main light-harvesting pigment. Strong excitonic interactions between peridinins, as well as between peridinins and chlorophylls (Chls) a, can be expected from the short inter-molecular distances revealed by the crystal structure. Different experimental approaches of nonlinear polarization spectroscopy in the frequency domain (NLPF) were used to investigate the various interactions between pigments in PCP of Amphidinium carterae at room temperature. Line shapes of NLPF spectra indicate strong excitonic interactions between the peridinins optically allowed S2 (1Bu+) states. A comprehensive subband analysis of the distinct NLPF spectral substructure in the peridinin region allows to assign perdinin subbands to the two Chls a in PCP having different S1 state lifetimes. Peridinin subbands at 487, 501 and 535 nm were assigned to the longer-lived Chl, whereas a peridinin subband peaking at 515 nm was detected in both clusters. Certain peridinin(s), obviously corresponding to the subband centered at 487 nm, show(s) specific (Coulombic?) interaction between the "optically dark" S1 (2Ag-) and/or intramolecular charge-transfer (ICT) state and S1 of Chl a. The NLPF spectrum, hence, indicates that this peridinin state is approximately isoenergetic or slightly above S1 of Chl a. A global subband analysis of absorption and NLPF spectra reveals that the Chl a Qy band consists of two subbands (peaking at 669 and 675 nm and having different lifetimes), confirmed by NLPF spectra recorded at high pump intensities. At the highest applied pump intensities an additional band centered at < 660 nm appears, suggesting - together with the above results - an assignment to a low-dipole moment S0 
S1/ICT-transition of peridinin.
Key Words:
PCP, carotenoids, nonlinear spectroscopy, optically dark states, peridinin, spectral heterogeneity
